Pup mounting for the Night Fly and P series are very different from a Deluxe. The P series and bolt-on Fly guitars use a standard mounting (maybe a base plate trim if needed). The Deluxe, Classic, Mojo, Supreme, Artist, etc. do not use legs on the pups, and they mount using screws that go through the coils in different locations depending on the model - Mojo (Duncan) and others (Dimarzio).
To fit Duncans in a Fly that came with Dimarzios requires cutting the legs off of a standard sized Duncan, trimming the baseplate to a perfect radius around the coil, and I think only one of the screw holes will line up for mounting. Its a tight fit.
I have had a handful of US Parkers (still have a Classic and a Mojo). It is not your typical DIY guitar 101.
If it is an older Deluxe with the original wiring (ribbons, circuit boards, looks like NASA designed it, etc), the tone is a no-load control - you get a bit more zing with the tone when wide open (more than your typical no-load in another guitar). The new Deluxe has standard pots (regular tone) and point-to-point wiring. Pickups mount the same in both.